Most conventional magnetic recording media are of the coated type and are produced by dispersing particles of magnetic oxides or ferromagnetic alloys such as .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, a Berthollide compound of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, and CrO.sub.2 in an organic binder such as a vinyl chloride/vinyl acetate copolymer, a styrene/butadiene copolymer, an epoxy resin or polyurethane resin, applying the resulting coating solution to a non-magnetic base, and drying the coating.
However, with the recent demand for higher density recording, researchers' attention has been drawn to binderless magnetic recording media of the thin metal film type which uses a thin ferromagnetic metal film as a magnetic recording layer, the film being formed by a vapor deposition such as vacuum deposition, sputtering and ion plating, or by a plating such as electroplating and electrolessplating. Accordingly, various efforts are being made to develop such products which can be used on a commercial basis.
Most conventional magnetic recording media of the coated type use a magnetic material of a metal oxide having a lower saturation magnetization than ferromagnetic metals; accordingly, attempts to achieve high density recording by using thinner magnetic recording media have inevitably resulted in a decreased signal output, thus putting limits on the uses of such thin magnetic films. Furthermore, processes of manufacturing such thin magnetic recording media of the coated type are complex and require large scale equipment for solvent recovery and pollution control.
In distinction, with binderless magnetic recording media, a very thin magnetic recording layer necessary for high density recording can be formed using a ferromagnetic metal having a higher saturation magnetization than the above magnetic oxides without using a non-magnetic material such as a binder and such recording media can be manufactured by a simple process.
It has been confirmed, both theoretically and empirically, that two requirements for a magnetic recording medium adapted for high density recording are high coercive force and thinness. Therefore, there is the expectation that binderless magnetic recording media can be made which are as thin as one-tenth the thickness of magnetic recording media of the coated type and which have a higher saturation flux density than the latter.
It is highly advantageous to form a thin magnetic film by vacuum deposition since this method yields the desired thin metal film rapidly, involves simple manufacturing steps and requires no effluent treatment (unlike plating). A method of vacuum deposition that is capable of yielding a magnetic film having a high coercive force and squareness ratio (desired for magnetic recording media) is what is called the "oblique vapor deposition" method, and such is described in U.S. Pat. Nos. 3,342,632, 3,342,633, etc. In this method, the greater the angle of incidence of a vapor stream that is directed onto the substrate, the greater the coercive force possessed by the resulting magnetic medium. However, a high angle of incidence also results in decreased deposition efficiency.
Other requirements that must be met by magnetic recording media using a thin ferromagnetic metal film are high resistance to corrosion and wear as well as consistent running properties. In the recording, the reproduction and the erase modes, the magnetic recording medium runs at high speed relative to the magnetic head(s) and it must run smoothly and consistently. At the same time, it should not be worn or broken by contact with the head and it is required that the recorded signal not be decreased or lost due to corrosion or other factors when the medium is stored. One method has been proposed to provide better durability and weatherproofness by use of a protective layer, but the protective layer cannot be made too thick if spacing loss between a recording head and a magnetic layer is to be prevented. It is therefore necessary to produce a magnetic film which itself is durable and proof against weather.
To solve these problems, Japanese Patent Application (OPI) No. 153707/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") proposes a method where the surface of a binderless mgnetic recording medium is coated with a polymeric material and Japanese Patent Application (OPI) No. 88704/78 discloses a method where at least one surface of a magnetic recording medium is coated with a surfactant having an HLB value of 5 to 30. Both of these methods are "wet" methods and use a solvent (that may be a pollutant) and they require special apparatus for mold inhibition. As a further disadvantage, when a web having a vapor deposited thin ferromagnetic film is taken out of the vacuum chamber in which deposition occurs or when it is coated with a polymeric material or a surfactant, the ferromagnetic film may be exposed to air and its surface may be oxidized or otherwise deteriorated, or pinholes that cause dropout may develop upon rubbing.
To solve these problems, Japanese Patent Application No. 82709/79 proposes a process for producing a magnetic recording medium characterized by vapor depositing a polymeric material on the surface of a magnetic recording medium; Japanese Patent Application No. 82710/79 describes a process for producing a magnetic recording medium characterized by vapor depositing an organic lubricant and a polymeric material on the surface of a magnetic recording medium simultaneously; and Japanese Patent Application (OPI) No. 154306/79 discloses a process for producing a wear resistant magnetic recording medium characterized by vapor depositing a layer of higher aliphatic acid on the surface of a magnetic recording medium. However, none of these methods are capable of providing a recording medium having satisfactory running properties and wear resistance. The method of Japanese Patent Application No. 82709/79 does not achieve adequate improvement in running properties and Japanese Patent Application No. 82710/79 and Japanese Patent Application (OPI) No. 15306/79 do not provide desired long-term stability.